A 2Vpp, 80-200MHz fourth-order continuous-time linear phase filter with automatic frequency tuning

A 2.3V CMOS 80-200MHz 4th-order continuous-time 0.05° equiripple linear phase filter with an automatic frequency tuning system is presented. An Operational Transconductance Amplifier (OTA) based on transistors operating in triode region is used to achieve a very wide linear input range and a wide transconductance tuning range. A system that integrates common-mode feedback (CMFB), common-mode feedforward (CMFF), and adaptive-bias into one circuit is introduced. The filter is based on Gm-C biquads and the experimental results in a 0.35µm process have shown a THD of less than-44dB for a 2Vppdifferential input with a single 2.3V power supply. The cutoff frequency (fc) is tunable in the range of 80-200MHz. The group delay ripple is less than 4% for frequencies up to 1.5fc. over the whole frequency tuning range. A simple automatic tuning system that adjusts the transconductance of the OTA is used to compensate the process parameter variations.